During takeoff and landing an aircraft can encounter hazardous flight conditions caused by windshear. Thunderstorms and rain showers often give rise to microbursts, which are intense localized downdrafts that spread along the ground, producing the phenomena known as windshear. Windshear is a defined change in wind direction and velocity within the air mass along an aircraft trajectory. When an aircraft enters windshear it encounters headwinds of growing strength that increase lift, enticing the pilot to reduce engine power and alter the aircraft attitude to maintain a desired flight path. As the aircraft transitions to the windshear center, the wind shifts to a severe downdraft that results in a strong downward force. This force can rapidly overcome available lift and lead to substantial loss in altitude. As the rapidly falling aircraft progresses to the back of the windshear, the wind direction shifts again, becoming an intense tailwind that further robs the aircraft of lift and increases rate of descent. The overall loss of altitude, coupled with the power reduction initiated when first entering windshear, can plunge an aircraft downward hundreds of feet in seconds, overwhelming the reaction time required to restore full engine power and establish a positive rate of climb.
Since windshears often occur within the first 300 meters of ground, the radar illuminates significant ground clutter when searching for the phenomenon. Microbursts can also contain relatively small amounts of moisture, resulting in low radar reflectivities and hence very low signal-to-clutter power ratios for associated windshear activity. Consequently, in order to estimate wind characteristics, clutter-related contributions to the radar returns must be filtered out, making the filtering process a key determinate of windshear detection performance.
The definition of clutter is application dependent. In general, any signal contributions not related to the signals of interest are classified as clutter. In the case of windshear detection, the signals of interest are the radar returns resulting from meteorological phenomena. All other signals, such as those obtained from stationary and moving objects located on the ground, are classified as clutter related. Noise, independent of both clutter related signals and signals of interest, is determined by such system limitations as effective antenna temperature and receiver noise figure.
Prior weather radars adapted for windshear detection process the radar returns to establish the Doppler spectrum thereof and then filter the Doppler frequency spectrum using a conventional stop-band filter. The filter is centered on a Doppler frequency that is related to the aircraft velocity and the antenna aspect angle. The filter center frequency must be precisely selected to prevent loss of desired signal. Even when an optimum value is chosen for the filter center frequency, spectral content of interest may be eliminated. Further drawbacks of the prior method are that the filter does not remove clutter resulting from stationary objects illuminated by antenna side lobes nor does it eliminate clutter resulting from moving discrete targets.